1. Field of the Invention
This invention relates to a welding apparatus and, more particularly, to a control system which allows overtravel but does not materially increase the force on the electrodes as a result of the overtravel.
2. Description of Related Art
One type of welding apparatus shown, for example, in Nyborg U.S. Pat. No. 3,191,000 includes a supporting structure, a movable member and an output member mounted for movement on the supporting structure, a first electrode mounted on the supporting structure and a movable electrode driven by the output member. Coupling means drivingly couples the output member to the movable member to move the movable electrode toward the fixed electrode so that a workpiece can be contacted by the electrodes and movement of the output member can be arrested. In addition, the coupling means allows the movable member to move relative to the output member to overtravel after movement of the output member is arrested by the workpiece. Accordingly, when the movable member is driven, the output member and the movable electrode are also driven. Means, such as a switch, is responsive to relative movement between the movable member and the output member for energizing the electrodes.
More specifically, the coupling means includes a coil compression spring that is preloaded a predetermined amount to thereby determine the force at which the weld is to be made. The differential motion between the movable member and the output member begins to occur when the force applied by the electrode to the workpiece just exceeds the preload on the spring. This differential motion or overtravel continues to compress the spring, and this increases the force applied to the workpiece by the electrodes above the desired force. Typically, the increase in force may be very substantial and is undesirable.
In an effort to solve the overtravel problem, a hard stop has been used to positively limit the amount of overtravel which can occur. With this construction, the overtravel still creates additional force on the weld, but by limiting the overtravel, it is hoped that the additional force will also be limited. However, the position of the hard stop must be adjusted for each different thickness of workpiece that is to be welded. This is time consuming and creates a possibility that the operator will forget to adjust, or incorrectly adjust, the position of the hard stop each time a different thickness workpiece is to be welded. In addition, the electrodes wear, and so to be effective, the position of the hard stop must be monitored periodically even if the welding apparatus is used continuously on workpieces of the same thickness.
Another attempted solution is to utilize an air cylinder to drive the movable electrode and a regulator set to produce the desired weld force. A problem with this system is that air pressure (weld force) and speed of operation are interdependent adjustments, making force changes more complicated.